In-mold decoration processes involve decorating articles as they are formed, in mold, of a heated plastic material being injected into a mold cavity. Usually a tape or strip of a decorating or protective material is automatically or manually advanced, pre-fed and positioned in the mold cavity at each molding cycle, interfacing therein with the plastic material as it is filled into the mold cavity, under heat and pressure. As the article is formed, the decorating material forms on the surface of the article and becomes an integral and permanent part of the article, through thermal transfer in the in-mold decoration process. Other molding processes such as thermal forming, blow molding and compression molding or stamping may also be used for the transfer of the decorating or protective material. Sometimes the process may also be called in-mold labeling or in-mold coating, and the transferable protective material may be called a thermal transfer overcoat or durable coat layer.
The decoration tape or strip usually comprises a carrier layer, a release layer, a durable layer, an adhesive or tie-coat layer and also a layer of decorative designs (metal or ink). After the injection molding transfer, the carrier layer and the release layer are removed, leaving the durable layer as the outmost layer. The durable layer therefore is an essential part of the decorative tape or strip as it serves as a protective layer with scratch resistance, mar or abrasion resistance and solvent resistance to protect the decorative designs and also the molded article.
An effective durable layer must meet certain criteria. For example, it needs to be a non-tacky or non-blocking coating to allow roll-up and also to be able to tolerate subsequent image forming conditions. Secondly it needs to be conformable during the injection molding process to adapt to the 3D shape of the molded article. In addition, an effective durable layer needs to be able to withstand a high shear force and high temperature polymer melt in the injection molding process. Furthermore, it needs to have excellent solvent and abrasion resistance to protect the decorative image during usage.
U.S. Pat. No. 5,795,527 discloses an in-mold decoration process in which a protective layer known as the hard coat layer is formed from a UV or electron beam curable resin. U.S. Pat. No. 5,955,204 discloses a transfer material which has an UV absorbing layer as a protective layer. The UV absorbing layer contains an acrylic polymer in which a skeleton having an UV absorbing property is introduced onto the molecular chains. These durable layers, however, tend to crack or show defects if the layer is fully cured before molding. This is especially the case if a sharp curvature or steep step height is a critical feature of the molded article. On the other hand, a partially cured or under-cured durable layer is often not sufficiently hard for subsequent processing steps (e.g., sputtering or vapor deposition and the patterning of a metallic decoration layer which is a very desirable feature for most applications).
U.S. Pat. Nos. 5,993,588 and 6,527,898 disclose a protecting layer partially cured by thermal energy followed by UV post cure after the molding process. These references allege that the compositions disclosed therein may represent an advancement of the protecting layers and provide protecting layers which have improved abrasion and chemical resistance and show less tendency to crack at the curved part of the surface of a molded article. However, such protecting layers have certain disadvantages. First of all, the synthesis and purification of a highly acrylated polymer with reactive hydroxyl group(s) for thermal crosslinking are expensive and time-consuming. In addition, in order to achieve a metallic decorative layer of high gloss, the partially thermal-cured durable layer preferably has a high heat distortion temperature and yet still has (1) high photoreactivity for the UV post curing at a high speed to achieve acceptable scratch resistance, solvent resistance and hardness, and (2) high flexibility for 3D contour molding. Unfortunately these requirements tend to be in conflict and as a result, the durable layer compositions often have a narrow process window for optimum metal deposition and the molding/post curing processes. The durable/protective layer and the in-mold decoration foil resulted from any of these methods tend to be brittle and show defects such as cracking and dust particles during handling and conversion. Furthermore, the thermal partial curing of the durable layer composition in a production coater tends to be difficult to control. A high speed crosslinking required for low cost production often results in a short storage stability or green time of the coating fluid. It is highly desirable that a high rate of crosslinking in the coater is achieved by a wider coating process window with a more stable composition.